Some image forming apparatuses that form an image electrophotographically include an optical scanning mechanism having a rotary polygon mirror which deflects a light beam emitted from a light source to illuminate a photosensitive member. A brushless motor is sometimes used as a driving motor for rotating the rotary polygon mirror. In a brushless motor, it is necessary to detect a position of a rotor to control energization timing for each coil. There has been proposed a known image forming apparatus, in which a plurality of Hall elements are placed in a vicinity of the rotor, and the position of the rotor is detected based on output signals of Hall elements.
In the known image forming apparatus, because of placement dispersion of the Hall elements with respect to the rotor, or the like, it is difficult to detect the position of the rotor accurately. Thus, the rotation control on the brushless motor may be unstable.